1. Field Of The Invention
This invention relates to nuclear magnetic resonance imaging and, in particular, it relates to an improved method and apparatus for correcting inaccuracies in magnetic resonance images which result from inhomogeneities in the magnetic field, and inhomogeneities that are produced by the subject, as well as by the imaging instrument.
2. Background Information
The advantageous use of noninvasive and nondestructive test procedures has long been known in both medicine and industrial applications. In respect of medical uses, it has also been known that limiting a patient's exposure to potentially damaging X-ray radiation may advantageously be accomplished through the use of other non-invasive imaging procedures such as, for example, ultrasound imaging and magnetic resonance imaging.
In a general sense, magnetic resonance imaging (MRI) involves providing bursts of radio frequency energy on a specimen positioned in a main magnetic field in order to induce responsive emission of magnetic radiation from the hydrogen nuclei or other nuclei. The emitted signal may be detected in such a manner as to provide information as to the intensity of the response and the spatial origin of the nuclei emitting the responsive magnetic signal. In general, the imaging may be performed in a slice, or plane or multiple or three-dimensional volume with information corresponding to the responsively emitted magnetic radiation being received by a computer which stores the information in the form of numbers corresponding to the intensity of the signal. The Pixel value is established in the computer by employing Fourier Transformation which converts the signal amplitude as a function of time to signal amplitude as a function of frequency. The signals are stored in the computer and may be delivered with or without enhancement to a video screen display such as a cathode-ray tube, for example, wherein the image created by the computer output will be presented through black and white presentations varying in intensity or color presentations varying in hue and intensity (and "saturation" or amount of "white" mixed in).
It has been known that magnetic resonance image intensity is dependent upon certain inherent physical properties of the tissues being investigated and timing intervals chosen by the user of the equipment. The physical properties of the tissues include the hydrogen density or density of the sensitive nucleus and two time factors which are known as T.sub.1 and T.sub.2. T.sub.1 which is also known as "T.sub.1 relaxation" is a measure of how long it takes the sample to regain its potential to produce a signal after a first pulse has caused it to respond to the pulsed RF excitation. This is sometimes considered as the time required to restore the longitudinal magnetization. T.sub.2 or "T.sub.2 relaxation" is a measure of the amount of time required for the magnetic resonance signal emitted by the radio frequency energy-excited proton to ideally dissipate to a point where it is generally imperceptible. At equilibrium, the transverse component of magnetization is at zero and the longitudinal component is equal to the initial magnetization. Decay to the former equilibrium is governed by the T.sub.2 relaxation and decay to the latter equilibrium is governed by the T.sub.1 relaxation. By properly selecting the timing intervals, the differences in hydrogen density or density of the sensitive nucleus, T.sub.1, and T.sub.2 values produce a difference in image intensity.
It has been known to use spin echo techniques in magnetic resonance imaging. In conventional spin echo imaging procedures, after an initial 90 degree pulse or general alpha-degree pulse, there are at predetermined intervals a 180 degree RF pulse and magnet which serve to refocus the transverse magnetization after the signal from the nuclei disappears to thereby cause the signal to reappear. The regenerated signal is referred to as a "spin echo". To the extent to which T.sub.2 relaxation has occurred prior to the generated spin echo, that portion of the signal is reduced.
As noted hereinbefore, utilization of nuclear magnetic resonance (hereinafter referred to as "NMR"), and particularly to forming an image utilizing NMR has become widespread in recent years. Although it has broad application, NMR imaging has become increasingly important in the medical arena where it is used to noninvasively inspect various parts of the human body.
It has been known to employ stereotactic procedures in neurosurgery applications. During such procedures, a rigid, three-dimensional reference frame is attached to the skull before and during the acquisition of volumetric images. The frame is used to guide the surgeon to the target area of the brain requiring surgery. The guidance is based on precise image measurements of geometrical coordinates of the target relative to the frame. The precision is such that the term "stereotactic" customarily implies ultimate precisional accuracy during surgery within one millimeter. Currently, stereotactic guidance is based on computerized tomography ("CT"). MRI has not been used in this field because the images obtained have heretofore suffered from geometric distortions greater than one millimeter which is an unacceptable degree of distortion. However, if such distortion were eliminated, MRI would be preferable to CT in many applications because it can provide superior soft tissue visualizations.
There have been some attempts to reduce distortion in MRI. For example U.S. Pat. No. 4,740,749 discloses an NMR imaging method based upon the modified spin warp method, but the method employs repeating the measurement of signals many times in order to decrease noise in the image. The method involves varying the intensity of the field or the time interval of the phase encoding gradients. The technique, however, is not satisfactory to reduce field inhomogeneities to the level of accuracy achieved by the present invention.
U.S. Pat. No. 4,728,890 discloses a motion artifact suppression technique involving rephasing the magnetic resonance signal components from moving tissues. The method includes a sequence during which several motion desensitization pulses are applied, and an algorithm for reconstruction of the image is provided.
U.S. Pat. No. 4,315,216 discloses an imaging system which applies magnetic fields to the body being examined nd which utilizes pulses which are not square pulses, instead, the pulse would perhaps be a distorted sinusoid. The resonance signal is sampled during that pulse.
U.S. Pat. No. 4,620,154 involves generating one image using a saturation-recovery method and generating a second image using an inversion-recovery method to create a third image from a ratio of the first and second images.
U.S. Pat. No. 4,825,159 discloses generating NMR images using a sequence involving reversal of gradient directions during successive sequences. However, it is not directed towards reduction of field inhomogeneities as is one of the objects of the present invention; instead, it is directed towards simultaneously obtaining two different images having differing diagnostic value.
U.S. Pat. No. 4,714,885 discloses an MRI technique allowing the operator to acquire data in a single scan which can be used for separate imaging of two spectrocomponents or for reducing chemical shift artifacts.
U.S. Pat. No. 4,818,941 discloses an improved imaging method using the spin warp technique involving a variation in the gradient field during sequence repetition. It involves the use of only one high frequency pulse during each sequence. In the next sequence, the pulse is shifted with respect to its distance from the echo signal. This method is directed towards generation of water and fat images and not towards the reduction in field inhomogeneities in the imaging technique.
U.S. Pat. No. 4,823,085 involves an imaging method which relates to the elimination of phase error in a single scan of the body. U.S. Pat. No. 4,745,364 also discloses a phase error correction technique.
U.S. Pat. No. 4,727,326 involves acquiring a first image in accordance with conventional methods and after slightly varying the orientater magnetic field, another image is acquired, then an image of a third type is formed by representing relative variations of the relaxation times between the first and the second image. This invention is directed towards constructing an image by discrimination of the intrinsic parameters of the tissues being examined.
U.S. Pat. No. 4,733,188 discloses an imaging method which involves repeating a sequence including changing the phase of a second gradient magnetic field which is periodically inverted in sense.
These patents do not achieve the accuracy of image which is achieved by the present invention in the time period or to the level of accuracy achieved by the present invention.
In spite of the foregoing attempts there remains a need for an improved method and related apparatus for providing MRI images which are nondistorted and which may be used during stereotactic neurosurgery and other procedures requiring extreme accuracy.